Achievements and Continued New Phase of the Large Helical Device

This paper reviews major achievements in the large helical device (LHD) project and discusses the prospects in the upcoming nearest future. The LHD is the largest magnetic confinement device among diversified helical systems and employs superconducting coils. These significant features have enabled comprehensive studies on steady-state net-current free plasmas with plasma parameters comparable to large tokamaks. The major achievements are highlighted by high beta (5.1%), high density (1.2×10m), high ion temperature (Ti of 5.6 keV at 1.6×10m), and steady-state operation (3200 s with 490 kW). This progress in physical parameters has elucidated the potential of net-current free helical plasmas for an attractive fusion reactor. The recent finding of an internal diffusion barrier has extended high density operation beyond 1×10m at the moderate magnetic field of 2.5 T. This achievement provides a novel scenario to a super high density reactor. Based on achievements that have resolved fundamental concerns in helical system, an upgrade plan to advance the LHD project to a new phase has just started. Major elements are a closed divertor system to improve particle control under high heating power and steady state, upgrade of heating capability and the use of deuterium gas.